This invention relates to two-piece child resistant closures which utilize a ratchet feature to screw on the closure and, more specifically, to a two-piece child resistant closure wherein a particular configuration and placement of the ratchet elements provides negligible reverse torque during undepressed counter-clockwise rotation of the outer closure member. Negligible reverse torque is defined as the torque caused by frictional contact of the outer and inner closure members when the undepressed closure is turned in the removal direction being insufficient to cause the undesired or accidental removal of the closure from the container.
Child resistant closures comprising two nested cap members are well known in the art. Typically, the inner and outer cap members of these closures are provided with cooperating sets of ratchet teeth which engage each other when the outer cap member is rotated in the direction to apply the closure to an associated container (usually the clockwise direction). When the outer cap member is rotated in the removal direction (typically counter-clockwise), these ratchet teeth cam or ratchet past each other on inclined surfaces, so that the closure generally cannot be removed by mere counter-clockwise rotation.
To provide positive engagement of the inner and outer closure members for removal, the known devices have employed sets of cooperating ratchet teeth or projections, which are normally held separated by a resilient or spring means. To remove these closures, the outer member must be variously squeezed, pushed, or otherwise manipulated to overcome the separating means, and bring the respective ratchet teeth into positive driving engagement.
The problem with known child resistant closures which utilize a ratchet feature to apply torque to screw on the closure is that this same feature must frictionally ride over itself under undepressed reverse torque of the closure to prevent unscrewing of the closure. In other words, all of the ratchet teeth on the inner and outer cap members must slide over one another. The frictional force created during this process is sometimes enough to unscrew the closure at low application torques. In other words, if the closure has not been relatively tightly placed on the container neck, it is possible that the closure may be accidentally and undesirably removed simply by virtue of the frictional force caused by the ratchet teeth sliding over one another during undepressed rotation of the outer cap member in the removal direction.
To assure that the ratchet teeth on the closure slide over one another without exerting sufficient torque or friction to unscrew the inner cap when the outer cap is turned in the removal direction, U.S. Pat. No. 3,776,407 to Cistone discloses the use of reduced thickness areas at the locations of the ratchet teeth of the outer ca to provide increased wall flexibility and a reduction in torque applied to the inner cap by the outer cap when the latter ratchets past the inner cap While the reduction in the skirt wall thickness is effective to increase the flexibility of the skirt in those areas and therefore generally accomplishes its intended purpose, the reduction of the thickness achieved by providing recessed areas in the outer wall circumferential surface has demonstrated some disadvantages, since the recessed areas necessarily result in a deviation from the desired overall circular configuration of the outer skirt surface.
Accordingly, it would be desirable to provide an improved closure assembly wherein the reverse torque resulting from the frictional force caused by the ratchet teeth sliding over one another is insufficient to affect removal of the closure.